Tuned radio frequency receivers generally comprise a plurality of radio frequency amplifiers followed by circuits which detect and amplify the signal. A three stage tuned radio frequency receiver includes an RF stage, a detector stage, and an audio stage. Two or three RF amplifiers, located in the radio frequency (RF) front end circuit, are used to establish the signal to noise ratio. For super-heterodyne architectures, the “RF front end circuit” may comprise: a matching circuit to allow the received energy from the antenna to get to the next stage; a band-pass filter (BPF) to knock down out-of-band jammers; another matching circuit at the input of the low-noise amplifier (LNA); the LNA; another matching network between the LNA output; and the receive (RX) mixer (down converter); and the RX mixer itself. The low-noise amplifier (LNA) operates to set the receive signal to noise ratio of the receiver by offering high gain and a very low noise figure (NF).
A characteristic measurement of the robustness of any (RF) front end circuit is the spurious free dynamic range (SFDR). The non-linear mixing process, which converts the inputted signal to intermediate frequency (IF) for low pass filtering, creates intermodulations (IMDs) that distort the received signal that may appear in the filter's bandpass region. In general, spurious free dynamic range is the usable dynamic range before spurious noise interferes or distorts the fundamental signal. It is a measure of how large the RF power of the interfering signals must be before IMDs appear within the passband of the receiver's low pass filter. The SFDR (Spurious Free Dynamic Range) indicates in decibels (dB) the ratio between the powers of the converted main signal and the greatest undesired spur. A formula used to evaluate the SPDR is: SFDR=20*log ([Fundamental]/[Highest Spurious]).
The traditional way to measure SFDR is to analyze the output of the intermediate frequency (IF) port displayed on a spectrum analyzer. A two-port system is appropriate when the device under test has both an analog RF input and analog IF output port as shown diagrammatically in FIG. 1. The two-port SFDR measurement setup (FIG. 1) references the analog RF input and analog intermediate frequency output ports of the mixer.
However, when the input is analog and the output is digital, a two-port two tone measurement set up cannot be used. With the increasing popularity of active RF identification (RFID), in the form of software controlled radio, traditional methods for measuring the SFDR of the receiver become impractical when the output is digital. Reduced form factors for RF circuits signify that in active RFID radio architectures, often the mixer and analog-to-digital converter (ADC) are housed within the same integrated circuit (IC) chip; therefore, the analog intermediate frequency (IF) port needed to perform the standard SFDR measurement may not be available. There is a need for an apparatus for making SFDR measurements of digital radio architectures when the user does not have access to an analog IF output.